An adhesive used for applications of flexible printed circuit (hereinafter referred to as an “FPC”) boards contains, as a curable component, a mixture of an epoxy resin and a curing agent for an epoxy resin. The adhesive further contains, for example, acrylonitrile-butadiene rubber (or Nitril-Butaadiene Rubber, hereinafter referred to as “NBR”) in order to improve the peel strength of the adhesive and to impart flexibility to the adhesive. Nitrile rubber including carboxyl groups crosslinkable with an epoxy resin is widely used as the above NBR in order to obtain favorable soldering heat resistance (see Patent Literature 1).
The adhesive composition used for the FPC applications contains a large amount of epoxy resin, and this causes a problem in that a large amount of unreacted epoxy resin etc. exudes during thermal compression molding and clogs openings provided in a cover lay, a reinforcing plate, etc.
Patent Literature 2 proposes a method for preventing exudation of an adhesive composition through openings by adding an ultraviolet (UV) curable resin to the adhesive composition and applying ultraviolet rays to the openings to cure the ultraviolet curable resin.
However, in the technique described in Patent Literature 2, the step of applying ultraviolet rays is additionally provided. Therefore, capital investment for the UV application step and special storage conditions for avoiding UV rays during storage are necessary.
Accordingly, to improve the storage conditions, the applicant used a thermosetting resin composition containing a prescribed acrylic copolymer, an epoxy resin, and a curing agent for an epoxy resin (Patent Literature 3). More specifically, the applicant used, as the curing agent, an organic acid dihydrazide having a prescribed particle diameter.